ESL systems consist of a large number of electronic labels, one for each merchandise item and a central server, together forming an ESL network. An electronic label displays the price and other information like name and/or price per kg of the corresponding the merchandise item. The central server has all the information for the electronic labels and has to communicate the information to be displayed to the corresponding electronic labels.
An ESL system is known from U.S. Pat. No. 6,108,367. The disclosed system is based on RF communication and batteries. Further, an ESL system is known from U.S. Pat. No. 6,715,676 which is based on infrared communication and the usage of solar cells and/or batteries.
EP1818864 discloses an ESL system making it possible to display for each commercial item the total inventory quantity indicating the number of commercial items that are not placed on the commercial item shelf but exist in the store as inventory.
With prior art ESL systems the network coordinator communicates to the electronic labels through a wireless link. Further, the network coordinator has a (wired or wireless) connection to a point-of-sale (POS) system that includes a POS terminal and a POS server with a database. The network coordinator can send from time to time a request to the POS system with respect to an updated price or a non-update status for a certain electronic label. Then, the POS system will send a reply to the network coordinator and in case of price update the update information will be forwarded to the electronic label in question.
When the network coordinator communicates to the POS system access has to be made to the POS database to find possible new price information for an electronic label in question. Different activities by the network coordinator and the POS system and the communication over the connection between the network coordinator and POS system, result in an unknown delay.
Energy harvesting with amorphous silicon solar cells allows power generation of around 3 μW/cm2 with an indoor light intensity level of 100 lux. By stacking such solar cells a supply current of several μA can be realized easily to store energy and creating a voltage of a few Volt with a cell area of several cm2 under certain typical indoor light conditions.
Nowadays LCD (liquid crystal display) techniques allow low cost small-size displays with low power consumption, in particular when the displayed information stays unchanged.
With ZigBee based designs for radio and networking, the functionality for an ESL network can be realized. ZigBee solutions are built around 2.4 GHz, “Wireless Medium Access, Control (MAC) and Physical Layer (PHY) Specifications for Low-Rate Wireless Personal Area Networks (LR-WPANs)”. Such 2.4 GHz radio designs are optimized for a minimum chip-area, a high integration level and minimum off-chip components to realize robust communication in the worldwide available 2.4 GHz band. These ZigBee solutions further attempt to limit the activity of the microprocessor and 2.4 GHz radio by sleep and standby modes. The power consumption can be reduced by turning off circuitry during certain time intervals. Further general background information can be found on www.zigbee.org.
The ZigBee application and network characteristics are described and built on top of the IEEE 802.15.4 radio MAC and PHY layers. The 802.15.4 MAC handles the radio channel medium access based on CSMA-CA (carrier sense multiple access with collision avoidance). ZigBee networks allow three network topologies: star, tree and mesh; and two operational modes: beaconed and non beaconed.
Non-beaconing is supported by star, tree and mesh networks. Here, an end-device is permanently allowed to access the medium and to send a message over the medium. A “parent” device can only send to an end-device an acknowledgement frame immediately after an end-device's data frame or a reply data frame after an end-device's data-request. Therefore, an end-device has self-control on its activity and takes the initiative to wake-up and to transmit a data frame (and to anticipate on a reply data frame), thus its activity allows event driven operation. Events which evoke transmission activity by the end-device can be initiated by an external trigger (like an alarm) but also by some external timer to take care of ‘alive’ traffic that is monitored at a central point. Since the end-device will initiate activity, it can stay in sleep mode until the next trigger occurs and a long period of inactivity can expire till the next ‘alive’ traffic trigger event. Therefore, such end-devices in non-beaconed devices enable a very low power operation and allow powering by an energy harvesting provision. However, with non-beaconing the coordinator and router have to be accessible permanently and consequently the receiver circuitry in a coordinator or router should be powered permanently.
In the 1970's several MAC protocols were developed for wireless networks. In the book J. L. Hammond, P. J. P. O'Reilly, Performance analysis of local computer networks, Addison-Wesley, 1986, ISBN 0-201-11530-1 (pp. 279-346) such protocols developed for both radio networks and wired networks are summarized. This publication refers also to various kinds of CSMA protocols and points to the network performance parameters such as normalized propagation delay (=contention window/packet transmission time) which is crucial to derive the throughput for a given load situation (which are often characterized by the normalized load=probability of packet arrival during a time interval equal to the packet transmission duration). One of these protocols is non-persistent CSMA at which station with a frame to transmit will sense the channel. If found busy, the station does not transmit.
The CSMA-CA protocol is designed to reduce the collision probability between multiple stations accessing the medium at the point in time where collisions would most likely occur. Collisions are most likely to happen just after the medium becomes free, following a busy medium. This is because multiple stations would have been waiting for the medium to become available again. Therefore, a random backoff arrangement is used to resolve medium contention conflicts. The collision avoidance portion of CSMA-CA is performed through a random backoff procedure based on binary exponential backoff algorithm. A station with a frame to transmit will compute a random backoff time before it senses the channel. If found busy, the station computes again a random backoff time. The random time is based on random number of slot times.
In the 1990's CSMA-CA was adopted as MAC protocol within IEEE 802.11 for wireless LANs.
IEEE 802.15.4 for wireless Personal Area Networks (PAN) adopted the CSMA-CA protocol. A station that has something to transmit, operates according a certain flow-diagram in the 802.15.4 standard and applies parameters as: macMinBE, macMaxBE, macMaxCSMABackoff, aMaxFrameRetries. IEEE 802.15.4 applies a slot time based on the time check if the medium is free and the receive-transmit turn around time.
Various aspects of the MAC functionality have been published related to the optimization of the random wait and to power management with respect to beacon based sleep modes. Some publications on the 802.15.4 MAC describe network performance aspects in various ways like by dynamic adaptation of parameters as macMinBE.